Insulated high voltage conductor with potential gradient equalization means



Dec. 30, 1969 G. LAURELL ETAL 3,487,455

INSULATED HIGH VOLTAGE CONDUCTOR WITH='POTENTIAL.;

GRADIENT EQUALIZATION MEANS Filed April 8, 1968 3 CORONA PREVfi/VT/VE fTAFE 10 ll 7 9 3 Q -INSZ/LAT/O/V sam- GOA/00670,? v I LAYER 8 f 3/4 /ca/v R0685? PASTE INVENTOR.

LARS s. LAURELL ARNE MANdT United States Patent Sweden Filed Apr. 8, 1968, Ser. No. 719,544 'Claims priority, application Sweden, Apr. 18, 1967, 5,350/67 Int. Cl. H011) 7/18 US. Cl. 174102 4 Claims ABSTRACT OF THE DISCLOSURE An insulated electrical conductor surrounded by a conducting body having a potential considerably different from that of the insulated conductor, with the conductor projecting outside of the conducting body. The projecting part of the insulating conductor is provided with a coating in electrical contact with the conducting body and having a substantially voltage dependent resistivity. This coating is formed by wrapping a tape of a thermoplastic resin or an elastomer containing silicon carbide particles, the successive layers of tape partially overlapping each other. A silicon rubber paste is applied to the part of the insulated conductor before the wrapping, and the wrapping is applied before the silicon rubber has had time to set.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to an insulated electrical conductor and to the prevention of corona at the ends thereof.

The prior art Swedish Patent No. 213,241 describes an insulated electrical conductor which is surrounded entirely or partly along its circumference by a conducting body having a completely different potential from the insulated conductor and which conductor projects outside this body, particularly a cable having an insulated conductor exposed outside the screen edge, which insulated conductor is provided on the projecting part with a coating in electrical contact with the conducting body and having pronounced voltage dependent resisitivity in the form of a wrapping of a tape of a thermoplastic resin or elastomer, for example, polyvinyl chloride, intermixed with silicon carbide. The tape is preferably wrapped around the insulated conductor with an overlap.

Such a coating has a strong non-linear current-voltage characteristic, so that the voltage over the active part of the coating within a large voltage range will be substantially constant and independent of the current strength through this part of the coating. The resistivity of the coating automatically assumes the most suitable value in each part regardless of the potential of the conductor and the coating thus automatically effects a satisfactory equalisation of the potential gradient when the voltage in the conductor alters within wide limits.

SUMMARY OF THE INVENTION The present invention relates to an improvement of the known insulated conductor which enables the conductor to withstand voltages more than twice as high as known conductors without the insulation on the exposed part being destroyed. The considerably increased resistance to voltage stresses is effected by filling the space between the insulated conductor and the voltage dependent coating consisting of a wrapping of overlapping turns of tape, with a silicon rubber paste cold-vulcanized in situ.

3,487,455 Patented Dec. 30, 1969 Even with the new method of applying the wrapping of the tape having voltage dependent resistivity on the insulated conductor, the application of the wrapping can be carried out without the use of heating means and without waiting for the binder in the coating to dry as is the case when it consists of a varnish. The coating can thus be finished immediately and in a very short time, of the order of magnitude of some minutes. Since cables are often joined and connected outdoors this feature of the invention has considerable importance.

The present invention thus relates to an electrical conductor which is surrounded entirely or partly along its circumference by a conducting body having a potential considerably different from that of the insulated conductor, with the conductor projecting outside this conducting body, particularly a cable having an insulated conductor exposed outside a screen edge, which insulated conductor is provided on the projecting part with a coating in electrical contact with the conducting body and having a substantial voltage dependent resistivity in the form of a wrapping of a tape of a thermoplastic resin or an elastomer, for example, polyvinyl chloride containing silicon carbide intermixed therein, which wrapping comprises layers of the tape overlapping partly adjacent layers of the tape, characterised in that spaces between the insulated conductor and the wrapping are filled with a silicon rubber paste which is cold-vulcanised in situ.

As silicon rubber paste may be used grades known per se and used commercially. For example a paste may be used with a hydroxypolymethylsiloxane having a viscosity of 5000-500,000 cp. at 25 C. as a binder. Such a paste is able to condense at room temperature, for example when intermixed with carboxylised metal salts or aliphatic organic metal salt complexes such as dibutyl-tin-dilaurate. Fillers are also usually included in the paste, such as silicon dioxide and titanium dioxide. In order to prevent polycondensation during storage, which occurs in enclosed packings, the paste also contains small quantities of an inhibitor, for example acetic anhydride. When the paste is applied the inhibitor evaporates, thus enabling polycondensation and the paste vulcanises to an elastomer.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail with reference to some embodiments chosen as examples, which are shown in the accompanying drawings where FIGURE 1 shows in side view a cable having one conductor FIGURE 2 a side view of a cable having three conductors and FIGURE 3 an axial section through the part A -A -A -A in the cables according to FIGURES 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the figures the metallic conductor 1 is provided with an insulation 2 which may consist, for example, of plasticised polyvinyl chloride or polyethylene, but even also of paper or the like. The insulated conductor, the cable part, has outside the conductor insulation a semi-conducting layer 3 which forms an equipotential surface on the outside of the cable and may consist, for example, of a thermoplastic resin or an elastomer containing frame-building carbon black, or of a graphitized paper. The thermoplastic resin or elastomer may consist, for example, of polyvinyl chloride, polyethylene, natural or synthetic rubber. The layer may consist of a seamless extruded casing or of a helically arranged tape. If the cable has several conductors, as in FIGURE 2, these are combined into one cable and provided with a common semi-conducting casing 4 which may consist of the same material and be manufactured in the same way as the semi-conducting layer on the individual insulated conductors. Around the layer 3 in the single-conductor cable or around the common casing 4 in the multi-conductor cable is a metallic sheath 5 consisting of helically applied wires or tape or longitudinally applied tapes or seamlessly applied metal such as lead or aluminium. The metal sheath may be externally protected by a casing 6 of, for example, polyvinyl chloride.

When installing a cable of the type described above, the metallic screen 5 is earthed, and the semi-conducting layer 3 of the insulated conductors thus also receives an earth potential, either through direct contact with the metal screen as is the case with the single-conductor cable according to FIGURE 1 or through contact with the metal screen 5 over the common casing 4, as is the case with the multi-conductor cable according to FIGURE 2. When installing the cable its ends are stripped and the individual insulated conductors freed as indicated in FIG- URES 1 and 2. In order to equalize the field concentration which arises at the edge of the screen during testing and operation, the insulation 2 is provided with a coating having a substantial voltage dependent resistivity in the form of a wrapping 7 of a tape of thermoplastic resin or elastomer, for example, polyvinyl chloride, containing silicon carbide. Before this tape is applied, a layer of cold-vulcanised silicon rubber paste is applied on the part of the insulation 2 extending from the edge 8 of the semiconducting layer 3 to a point 9 in the vicinity of where the wrapping 7 is intended to end (FIG. 3). This is applied all round the insulation. The paste may, for example, consist of the type described earlier with a hydroxypolymethylsiloxane having a viscosity of 5000-500,000 cp. at 25 as binder (for example RTV 102, General Electric Corp. or Silastic RTV 732, Dow Corning Corp.). Immediately after the paste has been applied on the insulation 2 and before it has had time to vulcanise, the tape 7 is wound around the insulation with overlap, for example 50% overlap between adjacent layers (for example 10, 11). Due to its plastic properties, therefore, the paste 12 fills all the spaces between the wrapping 7 and the insulation 2 which might arise during the wrapping. As soon as the wrapping 7 has been applied the cable can be finally assembled. Thus, it is not necessary to wait until the paste has vulcanised, which can take place after boxes or the like have been assembled. The time for vulcanisation is a few hours when the paste is applied in the manner described.

The tape 7 is described in more detail in Swedish Patent No. 213,241, to which reference is made for the composition of the tape 7. It may, for example be manufactured from a mixture of 67 parts by weight polyvinyl chloride, 33 parts by weight dioctyl phthalate, 1 part by weight white lead and 150 parts by weight silicon carbide of a particle fraction having an average particle size of 0.037 mm. and approximately 90% of the weight of the fraction within the particle sizes 0.02-about 0.07 mm.

The percentage by volume of silicon carbide in this thermoplastic mixture is approximately 38%. From the mixture obtained a tape is rolled using rollers having a temperature of 160 C., the tape having a thickness of approximately 0.3 mm. and a width of approximately 20 mm., which is then applied according to FIGURE 3. For a kv. resin-insulated high voltage cable, the length of the coating may be approximately 7 cm.

Instead of polyvinyl chloride, for instance polyethylene, polybutene and natural or synthetic rubber may be used in the tape 7. The silicon carbide content in the tape in the wrapping may suitably be between about 10-about 60, preferably 25-50 percent by volume. The silicon carbide may suitably consist of particle fractions having average particle sizes below 0.15 mm., preferably 4 with average particle sizes within the range 0.025 to 0.07 mm.

We claim:

1. Insulated electrical conductor which is surrounded at least partly along its circumference by a conducting body having a potential considerably different from that of the insulated conductor, the conductor projecting outside this conducting body, which insulated conductor is provided on the projecting part with a coating in electrical contact with the conducting body and having a substantially voltage dependent resistivity in the form of a wrapping of a tape of a substance selected from the group consisting of thermoplastic resins and elastomers, containing silicon carbide intermixed therein, characterized in that spaces between the insulated conductor and the wrapping are filled with a silicon rubber paste which has been cold-vulcanized in situ.

2. Method of manufacturing an insulated electrical conductor which is surrounded at least partly along its circumference by a conducting body having a potential considerably different from that of the insulated conductor, the conductor projecting outside this conducting body, comprising the steps of applying a layer of a cold vulcanizable silicon rubber paste to the projecting portion of the insulated conductor and, before the rubber is vulcanized, applying a wrapping of a tape selected from the group consisting of thermoplastic resins and elastomers containing silicon carbide intermixed therein to the silicon rubber paste along the insulated conductor and also applying the wrapping in electrical connection with the conducting body.

3. Cable comprising an insulated electrical conductor which is surrounded at least partly along its circumference by a conducting screen having a potential considerably different from that of the insulated conductor, the conductor projecting outside an edge of this conducting screen which insulated conductor is provided on the projecting part with a coating in electrical contact with the conducting screen and having a substantially voltage dependent resistivity in the form of a wrapping of a tape of a substance selected from the group consisting of thermoplastic resins and elastomers, containing silicon carbide intermixed therein, characterised in that spaces between the insulated conductor and the wrapping are filled with a silicon rubber paste which has been coldvulcanized in situ.

4. Method of manufacturing a cable comprising an insulated electrical conductor which is surrounded at least partly along its circumference by a conducting screen having a potential considerably different from that of the insulated conductor, the conductor projecting outside an edge of this conducting screen, comprising the steps of applying a layer of a cold vulcanizable silicon rubber paste to the projecting portion of the insulated conductor and, before the rubber is vulcanized, applying a wrapping of a tape selected from the group consisting of thermoplastic resins and elastomers, containing silicon carbide intermixed therein to the silicon rubber paste along the insulated conductor and also electrically connecting the Wrapping with the conducting screen.

E. A. GOLDBERG, Primary Examiner US. Cl. X.R. 

